In a Long Term Evolution (LTE) Rel-8/9/10 system, Physical Downlink Control Channels (PDCCHs) are transmitted in each radio sub-frame, and referring to FIG. 1, PDCCHs are typically transmitted in first N Orthogonal Frequency Division Multiplexing (OFDM) symbols of a sub-frame, where possible values of N are 1, 2, 3 and 4, and N=4 is allowable to occur only in a system with the system bandwidth of 1.4 MHz. The first N OFDM symbols herein are referred to as a “legacy PDCCH region”, and also referred to as a control region.
The control region for transmitting PDCCHs in the LTE Re1-8/9/10 system is logically divided into Control Channel Elements (CCEs), where a CCE is composed of nine Resource Element Groups (REGs), and the REGs belonging to a CCE are mapped throughout the whole bandwidth range by using the method based on interleaving of the REGs. An REG is composed of four Resource Elements (REs) duplicated in the time domain and adjacent in the frequency domain, where REs for transmitting common reference symbols are not included in the REs of which the REG is composed, and a particular definition of an REG is as illustrated in FIG. 2. Reference can be made to the description in the standard 36.211 for the particular definition of the REG and the mapping of the CCE to the REGs.
Downlink Control Information (DCI) is also transmitted in CCEs, and DCI for a User Equipment (UE) can be transmitted in M logically consecutive CCEs, where possible values of M in the LTE system are 1, 2, 4 and 8, each of which is referred to as an aggregation level of CCEs. The UE performs PDCCH blind detection in the control region to search for DCI transmitted for the UE, where the blind detection refers to that a decoding attempt is made for different DCI formats and aggregation levels of CCEs by using a Radio Network Temporary Identity (RNTI) of the UE, and if the decoding operation is correct, then DCI for the UE is received. The LTE UE needs to perform the blind detection on the control region in each downlink sub-frame in a non-Discontinuous Reception (non-DRX) state to search for DCI.
PDCCHs for a relay system, referred to as R-PDCCHs, are defined in the LTE-10 system, and the R-PDCCHs occupy the region for Physical Downlink Shared Channels (PDSCHs). FIG. 3 illustrates a particular structural diagram of R-PDCCH and PDSCH resources, where the R-PDCCH is used by a base station to transmit control signaling to a relay, and the legacy PDCCH region defined in the LTE Rel-8/9/10 system, is also referred to as a control region.
Resources occupied by the R-PDCCHs are configured in high-layer signaling, where Physical Resource Block (PRB) pairs occupied by the R-PDCCHs can be consecutive or inconsecutive.
A PRB is a resource unit composed of a slot in the time domain and a Resource Block (RB) in the frequency domain, where a slot includes seven consecutive OFDM symbols with a normal Cyclic Prefix (CP) or six consecutive OFDM symbols with an extended CP. Taking the normal CP as an example, an RB is composed of twelve sub-carriers consecutive in the frequency domain. Correspondingly a PRB pair is a resource unit composed of two slots in a sub-frame in the time domain and an RB in the frequency domain. As per the relevant definition of the search space of R-PDCCHs, there is no public search space of R-PDCCHs but a relay-specific R-PDCCH search space. Downlink (DL) grant signaling and Uplink (UL) grant signaling are transmitted through the Time Division Multiplexing (TDM) mode, where:
The DL grant is transmitted in a first slot in which the relay detects the DCI format 1A and a DCI format related to the downlink transmission mode; and
The UL grant is transmitted in a second slot in which the relay detects the DCI format 0 and a DCI format related to the uplink transmission mode.
Also two transmission modes are defined for transmission of R-PDCCHs respectively as an interleaving mode and a non-interleaving mode, where:
In the interleaving mode, the definition of PDCCHs, the aggregation levels and the CCE as the unit in the LTE Rel-8/9/10 system are also applied, where each CCE is composed of nine REGs, and a CCE is mapped to REGs also through interleaving as defined for PDCCHs; and
In the non-interleaving mode, the unit of the aggregation levels is a PRB, and there is a specific mapping relationship between resources occupied by candidate channels in the search space and the order of PRBs.
Referring to FIG. 4, FIG. 4 illustrates configuration diagrams of Channel State Information-Reference Signals (CSI-RS's) in the LTE Rel-10 system, where the numbered RE locations are the resource locations at which transmission of a CSI-RS may be configured in the system, and transmission modes of CSI-RS's include a 2-port multiplex mode, a 4-port multiplex mode and an 8-port multiplex mode. Each terminal is configured separately with the number of ports and resource locations of CSI-RS's, which may result in such a situation that different terminals occupy different resource locations. Moreover each terminal can further be configured with a Zero Power CSI-RS (i.e., CSI-RS at zero power), where the configuration is performed in accordance with the 4-port multiplex mode, and no signal is transmitted at a resource location corresponding to the Zero Power CSI-RS, for example, if a 4-port CSI-RS pattern is configured to be the Zero Power CSI-RS, then it indicates that the terminal considers that no Physical Downlink Shared Channel (PDSCH) data is transmitted at this RE location. As can be seen from FIG. 4, in a PRB pair, the size of resources capable of bearing the PDSCH transmission may vary dependent upon different configurations of the CSI-RS or Zero Power CSI-RS.
In the discussion of the Enhanced-PDCCH (E-PDCCH) in the LTE Rel-11 system, it has been determined that there are two transmission modes for the E-PDCCH, i.e., frequency-domain consecutive (localized) and inconsecutive (distributed) transmission modes, which are applicable to different scenarios. Typically the localized transmission mode is generally applicable to such a scenario that the base station can obtain comparatively precise channel information fed back from the terminal and the interference from an adjacent cell will not vary sharply from one sub-frame to another, where according to Channel State Information (CSI) fed back from the terminal, the base station selects consecutive frequency resources with comparatively good quality to transmit E-PDCCHs for the terminal and performs the pre-coding/beam-forming process to improve the transmission performance. In the case that no channel information can be obtained accurately or the interference from an adjacent cell sharply varies from one sub-frame to another and is unpredictable, E-PDCCHs need to be transmitted in the distributed transmission mode, that is, they are transmitted over frequency resources inconsecutive in frequency to thereby obtain a frequency diversity gain. FIG. 5 and FIG. 6 illustrate examples of E-PDCCH transmission in the localized transmission mode and in the distributed transmission mode respectively, where the transmission of one DCI occupies four PRB pairs.
There are several possible definitions of the E-PDCCH resource as follows:
A. A PRB pair is divided into a specific number N of E-REGs/E-CCEs which may be the same or different in size.
B. A PRB pair is divided into an integer number of E-REGs/E-CCEs dependent upon the configuration of the system (e.g., the configuration of a legacy PDCCH region, a CRS, a DMRS, a CSI-RS/Zero Power CSI-RS, etc.), where the number of E-REGs/E-CCEs is determined based on the configuration of the system and may vary from one sub-frame to another.
C. RE resources available in a PRB pair are divided into an integer number of E-REGs, each of which includes the same number of REs, and an E-CCE is composed of a specific number of E-REGs.
With the assumption of the configuration described above, for the case that a PRB pair is divided into a specific number N of E-REGs/E-CCEs, E-PDCCHs at the same aggregation level may have varying demodulation performance from one sub-frame to another. For example, when a PRB pair includes four E-CCEs all the time, if reference signals of the system are transmitted in more REs in the sub-frame 1, then there will be less available REs in which PDCCHs are transmitted; and on the contrary, if reference signals of the system are transmitted in less REs in the sub-frame 2, then there will be more available REs in which E-PDCCHs can be transmitted. Therefore, if the two sub-frames described above are configured with the same specific number N of E-REGs for the E-PDCCH transmission, then there will be a sharp difference between the numbers of REs included in E-CCEs configured respectively in the two sub-frames. For example, if the number of available REs to transmit E-PDCCHs in the sub-frame 1 is 15, and the number of available REs to transmit E-PDCCHs in the sub-frame 2 is 20, then there are 15 REs per E-REG in the sub-frame 1 and 30 REs per E-REG in the sub-frame 2 given N=1, and apparently this E-PDCCH transmission mode may degrade greatly the performance of E-PDCCHs.